End-surface reflection type surface acoustic wave filter

ABSTRACT

An end-surface reflection type surface acoustic wave filter is capable of increasing an attenuation amount outside a pass band while insertion loss characteristics are not seriously deteriorated. The filter is a longitudinally coupled resonator type surface acoustic wave filter using an SH type surface acoustic wave, which has first and second grooves formed in a piezoelectric substrate at the top surface thereof so as to be substantially parallel to each other and spaced from each other by a predetermined distance. In addition, IDTs which are provided between the grooves for defining the longitudinally coupled resonator type surface acoustic wave filter, reflection end-surfaces disposed on side surfaces of the first and the second grooves at the IDT sides, and one of a resin-coating layer and a protective layer made of SiO 2 , are provided on the top surface of the piezoelectric substrate. The resin-coating layer is arranged to cover at least a region at which the IDTs are located and at least one of the first and the second grooves so as to intrude into one of them, and the protective layer is arranged so as to cover the IDTs.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to surface acoustic wave filters for use,for example, as a band-pass filter, and more particularly, the presentinvention relates to an end-surface reflection type surface acousticwave filter which utilizes an SH type surface acoustic wave, such as aBGS wave or a Love wave.

2. Description of the Related Art

A known end-surface reflection type surface acoustic wave deviceutilizing an SH type surface acoustic wave such as a BGS(Bleustein-Gulyaev-Shimizu) wave or a Love wave is a longitudinallycoupled resonator type surface acoustic wave filter. In an end-surfacereflection type surface acoustic wave device, a surface acoustic wave isreflected between two end-surfaces opposing each other. Accordingly,reflectors are not necessary and as a result, the surface acoustic wavedevice such as a surface acoustic wave filter can be miniaturized.

FIG. 14 is a perspective view of a conventional end-surface reflectiontype and longitudinally coupled resonator type surface acoustic wavefilter. A longitudinally coupled resonator type surface acoustic wavefilter 101 has a piezoelectric substrate 102. In the piezoelectricsubstrate 102 on the top surface thereof, a first groove 102 a and asecond groove 102 b are formed so as to be parallel to each other andare spaced from each other by a predetermined distance. Between thegrooves 102 a and 102 b, two interdigital transducers (IDTs) 103 and 104are provided for constituting the longitudinally coupled resonator typesurface acoustic wave filter.

In addition, side surfaces of the first and the second grooves 102 a and102 b at the sides at which the IDTs 103 and 104 are located constitutereflection end-surfaces for reflecting an SH-type surface acoustic wave.

In the longitudinally coupled resonator type surface acoustic wavefilter 101 described above, since reflectors are not necessary,miniaturization can be achieved. However, it has been required that anattenuation amount outside a pass band be further increased but this hasnot been possible previously.

SUMMARY OF THE INVENTION

In order to overcome the problems described above, preferred embodimentsof the present invention provide an end-surface reflection type surfaceacoustic wave filter which greatly increases an attenuation amountoutside the pass band and significantly improves the degree ofselectivity.

In accordance with a first preferred embodiment of the presentinvention, an end-surface reflection type surface acoustic wave filterincludes a piezoelectric substrate having first and second reflectionend-surfaces disposed substantially parallel to each other and spacedfrom each other by a predetermined distance, a top surface, a bottomsurface, first and second piezoelectric substrate portions extendingfrom the bottom ends of the first and the second reflection end-surfacesto the outside, and first and second grooves or first and second recessportions open to the outside, the first and the second reflectionend-surfaces and the first and the second piezoelectric substrateportions defining the first and the second grooves or the first and thesecond recess portions, respectively, a plurality of interdigitaltransducers disposed on the top surface of the piezoelectric substratebetween the first and the second grooves or between the first and thesecond recess portions, and a resin-coating layer which is provided onthe top surface of the piezoelectric substrate so as to cover at leastthe plurality of interdigital transducer and regions at which the firstand the second grooves or the first and the second recess portions aredisposed and which resin-coating layer includes a flexible resin. In theend-surface reflection type surface acoustic wave filter describedabove, the resin-coating layer partially intrudes into at least one ofthe first and the second grooves or at least one of the first and thesecond recess portions.

In accordance with a second preferred embodiment of the presentinvention, an end-surface reflection type surface acoustic wave filterincludes a piezoelectric substrate having first and second reflectionend-surfaces disposed substantially parallel to each other and spacedfrom each other by a predetermined distance, a top surface, a bottomsurface, first and second piezoelectric substrate portions extendingfrom the bottom ends of the first and the second reflection end-surfacesto the outside, and first and second grooves or first and second recessportions open to the outside, the first and the second reflectionend-surfaces and the first and the second piezoelectric substrateportions defining the first and the second grooves or the first and thesecond recess portions, respectively, a plurality of interdigitaltransducers disposed on the top surface of the piezoelectric substratebetween the first and the second grooves or between the first and thesecond recess portions, and a resin-coating layer which is provided onthe top surface of the piezoelectric substrate so as to cover at leastthe plurality of interdigital transducers and regions at which the firstand the second grooves or the first and the second recess portions aredisposed and which resin-coating layer includes a flexible resin. In theend-surface reflection type surface acoustic wave filter describedabove, the resin-coating layer is arranged so as to cover approximately20% or greater of an area of the top surface of the piezoelectricsubstrate where the plurality of interdigital transducers are disposed.

In the end-surface reflection type surface acoustic wave filterdescribed above, the flexible resin may preferably have a Shore hardnessof about 30 or less. When a resin having a Shore hardness of about 30 orless is used, while deterioration of insertion loss characteristics issuppressed, undesired spurious can be effectively prevented.

In the end-surface reflection type surface acoustic wave filterdescribed above, as the flexible resin described above, a gel resin maybe used. Consequently, while deterioration of insertion losscharacteristics is suppressed, undesired spurious can be effectivelyprevented.

In addition, according to another preferred embodiment of the presentinvention, as the flexible resin described above, a resin having adensity of approximately 1.2 g/cm³ or less or, more preferably,approximately 1.0 g/cm³ or less after being cured may be used.Consequently, while deterioration of insertion loss characteristics issuppressed, undesired spurious can be effectively presented.

In the end-surface reflection type surface acoustic wave filterdescribed above, as the flexible resin described above, a resin having aYoung's modulus of about 1 MPa or less after being cured may be used.Consequently, while deterioration of insertion loss characteristics issuppressed, undesired spurious can be effectively prevented.

In the end-surface reflection type surface acoustic wave filterdescribed above, as the flexible resin described above, a resin having alinear expansion coefficient of approximately 1.9×10⁻⁴ (1/° C.) orgreater or, more preferably, approximately 2.3×10⁻⁴ (1/° C.) or greaterafter being cured may be used. Consequently, while deterioration ofinsertion loss characteristics is suppressed, undesired spurious can beeffectively prevented.

As the flexible resin described above, a gel resin having a density ofabout 1.0 g/cm³ or less, a Young's modulus of about 1 MPa or less, and alinear expansion coefficient of about 2.3×10⁻⁴ (1/° C.) or greater afterbeing cured is more preferably used.

As the flexible resin described above, for example, a silicone rubber,an epoxy resin, or a urethane rubber may be used, and more preferably, asilicon rubber is used.

In accordance with a third preferred embodiment of the presentinvention, an end-surface reflection type surface acoustic wave filterincludes a piezoelectric substrate having first and second reflectionend-surfaces disposed substantially parallel to each other and spacedfrom each other by a predetermined distance therebetween, a top surface,a bottom surface, first and second piezoelectric substrate portionsextending from the bottom ends of the first and the second reflectionend-surfaces to the outside, and first and second grooves or first andsecond recess portions open to the outside, the first and the secondreflection end-surfaces and the first and the second piezoelectricsubstrate portions defining the first and the second grooves or thefirst and the second recess portions, respectively, a plurality ofinterdigital transducers disposed on the top surface of thepiezoelectric substrate between the first and the second grooves orbetween the first and the second recess portions, and a layer whichincludes SiO₂ and is arranged so as to cover the plurality ofinterdigital transducers.

In addition, in the end-surface reflection surface acoustic wave filterin accordance with the third preferred embodiment of the presentinvention, when a wavelength of a surface acoustic wave which is to beused is represented by λ, and an electrode thickness of the interdigitaltransducer is represented by H, H/λ≦0.06 is preferably satisfied, andH/λ≦0.045 is more preferably satisfied. In the case described above,deterioration of insertion loss characteristics can be effectivelyprevented.

In the end-surface reflection type surface acoustic wave filters inaccordance with the first to the third preferred embodiments of thepresent invention, the grooves or the recess portions each preferablyhave a depth that is substantially equal to or larger than a wavelengthof an SH type surface acoustic wave which is to be used. Propagation ofan SH type surface acoustic wave is primarily performed at a depth thatis substantially equivalent to or smaller than the wavelength thereoffrom the surface of the piezoelectric substrate. Accordingly, when eachof the grooves or the recess portions has a depth in accordance therange described above, the SH type surface acoustic wave which is to beused is reliably reflected on the end-surfaces, and hence a surfaceacoustic wave filter having superior properties can be provided.

The structures of the end-surface reflection type surface acoustic wavefilters in accordance with the first to the third preferred embodimentsof the present invention are not specifically limited, and it may be aladder type filter including an end-surface reflection resonator, alongitudinally coupled resonator filter, or a transversely coupledresonator filter.

Other features, elements, characteristics and advantages of the presentinvention will become more apparent from the following detaileddescription of preferred embodiments thereof with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing an end-surface reflection type andlongitudinally coupled resonator type surface acoustic wave filteraccording to a preferred embodiment of the present invention;

FIG. 2 is a perspective view showing the longitudinally coupledresonator type surface acoustic wave filter shown in FIG. 1 after aresin-coating layer including a flexible resin is removed therefrom;

FIG. 3 is a partial, cutaway cross-sectional view for illustrating amajor portion of the longitudinally coupled resonator type surfaceacoustic wave filter according to the preferred embodiment shown in FIG.1;

FIG. 4 is a graph showing attenuation amount versus frequencycharacteristics of longitudinally coupled resonator type surfaceacoustic wave filters according to a preferred embodiment of the presentinvention and a conventional example;

FIG. 5 is a graph showing the relationship between spurious suppressionand a ratio of resin coating area;

FIG. 6 is a graph showing the relationship between insertion loss and anintrusion amount of a resin-coating layer into a first groove;

FIG. 7 is a graph showing the relationship between an intrusion amountof a resin-coating layer into the first groove and spurious suppressionoutside the pass band;

FIG. 8 is a graph showing the relationship among density of a curedresin-coating layer, insertion loss, and spurious suppression;

FIG. 9 is a graph showing the relationship among Young's modulus of acured resin-coating layer, insertion loss, and spurious suppression;

FIG. 10 is a graph showing the relationship among a linear expansioncoefficient of a cured resin-coating layer, insertion loss, and spurioussuppression;

FIG. 11 is a perspective view showing a modified example of alongitudinally coupled resonator type surface acoustic wave filteraccording to a preferred embodiment of the present invention;

FIG. 12A is a partial, cutaway cross-sectional view of another modifiedexample of a longitudinally coupled resonator type surface acoustic wavefilter according to a preferred embodiment of the present invention;

FIG. 12B is a graph showing attenuation amount versus frequencycharacteristics of said another modified example of a preferredembodiment of the present invention and that of a surface acoustic wavefilter which is prepared for comparison therewith;

FIG. 13 is a graph showing the relationship between insertion loss andIDT thickness of the surface acoustic wave filter shown in FIG. 12A; and

FIG. 14 is a perspective view showing a conventional longitudinallycoupled resonator type surface acoustic wave filter.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Hereinafter, the present invention will be described in detail withreference to particular preferred embodiments.

FIG. 1 is a perspective view of a longitudinally coupled resonator typesurface acoustic wave filter 1 according to a preferred embodiment ofthe present invention. In the longitudinally coupled resonator typesurface acoustic wave filter 1, a substantially rectangularpiezoelectric substrate 2 is preferably used. The piezoelectricsubstrate 2 is preferably made of a piezoelectric single crystal, suchas LiTaO₃ or LiNbO₃, or a piezoelectric ceramic such as lead titanatezirconate. However, other suitable materials may be used for thesubstrate 2. In addition, the piezoelectric substrate 2 may be alaminate formed of piezoelectric thin-films, such as ZnO thin-films,provided on an insulating substrate made of sapphire or other suitablematerial.

FIG. 2 is a perspective view showing the longitudinally coupledresonator filter 1 after a flexible resin-coating layer 3 provided onthe top surface of the piezoelectric substrate 2 is removed. As shown inFIG. 2, at a top surface 2 a of the piezoelectric substrate 2, first andsecond grooves 2 b and 2 c are arranged substantially parallel to eachother and are spaced from each other by a predetermined distancetherebetween. The depth of each of the grooves 2 b and 2 c is such thatthey grooves do not to extend to a bottom surface 2 d. Side surfaces ofthe grooves 2 b and 2 c at the internal sides thereof define first andsecond reflection end-surfaces 2 b ₁ and 2 c ₁. The depth of each of thefirst and the second grooves 2 b and 2 c is preferably substantiallyequal to or larger than a wavelength of a surface acoustic wave which isto be used.

In addition, as shown in FIG. 11, instead of the first and the secondgrooves 2 b and 2 c, first and second recess portions 2 x and 2 y may beformed. That is, in a piezoelectric substrate 2A shown in FIG. 11, firstand second reflection end-surfaces 2 b ₁ and 2 c ₁ are substantiallyparallel to each other and are spaced from each other by a predetermineddistance, and piezoelectric substrate portions 2 x ₁ and 2 y ₁ extendingfrom the bottom ends of the reflection end-surfaces 2 b ₁ and 2 c ₁ tothe outside are provided, thereby defining the first and the secondrecess portions 2 x and 2 y which are open to the outside. In thepresent invention, as described above, instead of the first and thesecond grooves, the piezoelectric substrate 2A having the first and thesecond recess portions open to the outside may be used.

Between the first and the second grooves 2 b and 2 c, longitudinallycoupled resonator type filter portions are defined. The longitudinallycoupled resonator type filter portion has a pair of IDTs 5 and 6disposed in the direction of surface acoustic wave propagation.

The longitudinally coupled resonator type surface acoustic wave filterof the present invention is not limited to the one-stage structuredescribed above, and a longitudinally coupled resonator type surfaceacoustic wave filter having at least a two-stage structure may be used.In accordance with the structure of a surface acoustic wave filter whichis to be provided, a plurality of IDTs is disposed on the top surface ofthe piezoelectric substrate 2.

One of the unique features of the longitudinally coupled resonator typesurface acoustic wave filter 1 of this preferred embodiment is that theflexible resin-coating layer 3 shown in FIG. 1 is arranged so as tocover the top surface 2 a of the piezoelectric substrate 2 describedabove. The flexible resin-coating layer 3 is arranged so as to cover theIDTs 5 and 6 and the first and the second grooves 2 b and 2 c. Inaddition, in this preferred embodiment, portions of the top surface ofthe piezoelectric substrate 2 at the external sides of the first and thesecond grooves 2 b and 2 c are also covered with the flexibleresin-coating layer 3. However, it is not necessarily required that theportions of the top surface of the piezoelectric substrate 2 describedabove be covered with the flexible resin-coating layer 3.

The flexible resin-coating layer 3 is preferably made of a resin havingflexibility so as not to suppress propagation of a surface acoustic wavewhich is excited when the longitudinally coupled resonator type surfaceacoustic wave filter 1 operates. As the resin described above, variousresins may be used, and in particular, a gel resin is preferably used.In addition, as the flexible resin described above, a silicone rubber,an epoxy resin, or a urethane rubber is preferably used, and inparticular, a silicone rubber is more preferably used. Preferablecharacteristics of a flexible resin may be described later in detailswith reference to various preferred embodiments.

In addition, as shown in FIG. 3, the resin-coating layer 3 is arrangedso as to intrude into the first groove 2 b.

In the longitudinally coupled resonator type surface acoustic wavefilter 1, since the flexible resin-coating layer 3 is arranged so as tocover the regions at which the IDTs 5 and 6 are located and the grooves2 b and 2 c and so as to intrude into the groove 2 b, an attenuationamount outside the pass band can be increased. Hereinafter, thispreferred embodiment will be described with reference to a specificexample.

As the longitudinally coupled resonator type surface acoustic wavefilter according to the preferred embodiment described above, apiezoelectric substrate, made of LiTaO₃, having outside dimensions ofapproximately 0.9 mm, 1.7 mm, and 0.38 mm of thickness was prepared, andon the top surface thereof, the IDTs 5 and 6 were formed. The totalnumber of pairs of electrode fingers of each of the IDTs 5 and 6 was 30and the wavelength therefor was about 21.8 μm. In addition, after theIDTs were formed, the first and the second grooves 2 b and 2 c eachhaving a width of about 0.17 mm and a depth of about 0.1 mm were formed.The longitudinally coupled resonator type surface acoustic wave filterthus formed was used as a conventional example, and as a longitudinallycoupled resonator type surface acoustic wave filter according to thepresent preferred embodiment, the filter described above was providedwith a flexible resin-coating layer 3 made of a gel silicone resin onthe top surface thereof.

Attenuation amount versus frequency characteristics of thelongitudinally coupled resonator type surface acoustic wave filtersaccording to the conventional example and the present preferredembodiment were measured. The results are shown in FIG. 4. The solidline in FIG. 4 shows the characteristics of the present preferredembodiment, and the dotted line shows the characteristics of theconventional example. In addition, the solid line P and the dotted lineQ shown in the central portion in FIG. 4 show the characteristics of thepresent preferred embodiment and the conventional example, respectively,which characteristics are represented in accordance with an expandedscale in the vertical axis at the right side.

As shown in FIG. 4, in the longitudinally coupled resonator type surfaceacoustic wave filter of the preferred embodiment, which is provided withthe flexible resin-coating layer 3, a loss is slightly increased in thepass band. However, it is understood that the attenuation amount outsidethe pass band can be significantly improved. The reason for this hasbeen considered that since the flexible resin-coating layer 3 isprovided so as to intrude into the first groove 2 b, reflection of anunnecessary wave between the reflection end-surfaces is prevented.

According to the results described above, the inventors of the presentinvention variously changed the coating area of the flexibleresin-coating layer 3 on the top surface of the piezoelectric substrate2 and measured the suppression degree of spurious indicated by thearrows A1 and A2 shown in FIG. 4. The results are shown in FIG. 5.

In this preferred embodiment, a ratio (%) of the resin coating area inthe horizontal axis shown in FIG. 5 is a ratio of the coating area ofthe resin-coating layer 3 to the area of the top surface of thepiezoelectric substrate 2 where the IDTs 5 and 6 are disposed, in otherwords, an area of the top surface of the piezoelectric substrate 2sandwiched by reflection end-surfaces 2 b ₁ and 2 c ₁.

As shown in FIG. 5, it can be understood that, with an increase in resincoating area of the insulating resin-coating layer 3, the spuriousresponses indicated by the arrows A1 and A2 are suppressed, and as aresult, it is understood that the attenuation amount outside the passband is greatly increased.

In addition, the relationship among the intrusion amount of the flexibleresin-coating layer 3 into the first and the second grooves, theinsertion loss within the pass band, and the above-described spuriouslevel outside the pass band was measured. The results are shown in FIGS.6 and 7.

An intrusion amount of zero shown in FIGS. 6 and 7 means that theflexible resin-coating layer 3 is provided only in the region at whichthe IDTs 5 and 6 are located, and an intrusion amount in the range ofabout 0% to about 100% means a ratio of a cross-sectional area of theflexible resin-coating layer 3 which intrudes into the groove 2 b to across-sectional area of the groove 2 b.

As shown in FIG. 6, as intrusion amount of the flexible resin-coatinglayer 3 into the groove increases, the insertion loss deteriorates.However, when the intrusion amount of the flexible resin-coating layer 3into the groove 2 b is about 50% or less, it is understood thatdeterioration of insertion loss characteristics can be suppressed toabout 2 dB or less. Accordingly, when the intrusion amount of theflexible resin-coating layer 3 into the groove 2 b, which is one of thegrooves, is about 50% or less, the insertion loss characteristic is notso much deteriorated. As shown in FIG. 7, it is understood that thespurious can be suppressed in the proximity of the pass band, and hencethe attenuation amount outside the pass band can be increased.

Accordingly, the flexible resin-coating layer 3 is preferably arrangedso that the intrusion amount thereof into one of the first and thesecond grooves 2 b and 2 c is at a ratio of about 50% or less.

Next, in the preferred embodiment described above, the inventors of thepresent invention variously changed materials used for forming theflexible resin-coating layer and measured the relationship among thedensity thereof, the insertion loss, and the suppression of spurious,i.e. spurious improvement degree indicated by the arrows A1 and A2 inFIG. 4. The results are shown in FIG. 8. The horizontal axis of FIG. 8indicates density of the flexible resin-coating layer, and in this case,the resin coating area of the flexible resin-coating layer was 100%. Inaddition, in FIG. 8, mark  indicates the result of the insertion lossin the case in which the resin-coating layer was made of an epoxy resin,mark + indicates the result of the spurious suppression in the abovecase in which the epoxy resin was used, mark ∘ indicates the result ofthe insertion loss obtained when a silicone resin was used, and mark xindicates the result of the spurious suppression in the above case inwhich the silicone resin was used.

In addition, the relationship among Young's modulus of a materialforming the flexible resin-coating layer, the insertion loss, and thespurious suppression is shown in FIG. 9. Marks , +, ∘, and x in FIG. 9are the same as those in the case described with reference to FIG. 8.

Furthermore, the relationship among a linear expansion coefficient(×10⁻⁴/° C.) of the flexible resin-coating layer, the insertion loss,and the spurious suppression is shown in FIG. 10. Marks , +, ∘, and xin FIG. 10 are the same as those in the case described with reference toFIG. 8.

As shown in FIG. 8, when the density of the cured flexible resin-coatinglayer is preferably about 1.2 g/cm³ or less and is more preferably about1.0 g/cm³ or less, it is understood that while the deterioration ofinsertion loss characteristics is suppressed, spurious can beeffectively prevented.

As in the case described above, as shown in FIG. 9, when the Young'smodulus of the cured flexible resin-coating layer is about 1 MPa orless, it is understood that while the deterioration of insertion losscharacteristics is suppressed, the spurious can be effectively preventedas in the case described above.

In addition, as shown in FIG. 10, when the linear expansion coefficientof the cured flexible resin-coating layer is preferably about 1.9×10⁻⁴(1/° C.) or greater and is more preferably about 2.3×10⁻⁴ (1/° C.) orgreater, it is understood that while the deterioration of insertion losscharacteristics is suppressed, the spurious can be effectivelyprevented. Accordingly, a preferable resin forming the flexibleresin-coating layer described above is a gel resin, and after beingcured, the resin described above preferably has a density of about 1.0g/cm³ or less, a Young's modulus of about 1 MPa or less, and a linearexpansion coefficient of about 2.3×10⁻⁴ (1/° C.) or greater.

In accordance with another preferred embodiment of the presentinvention, a protective layer 11 made of SiO₂ is provided on the topsurface 2 a of the piezoelectric substrate 2 so as to cover the IDT 5.In FIG. 12A, a portion of the top surface 2 a at which the IDT 6 isprovided is not shown. However, the protective layer 11 is also arrangedso as to cover the IDT 6.

In FIG. 12B, attenuation amount versus frequency characteristics of alongitudinally coupled resonator type surface acoustic wave filterprovided with the protective layer 11 described above are shown by thesolid line. For comparison, attenuation amount versus frequencycharacteristics of a longitudinally coupled resonator type surfaceacoustic wave filter, having the same structure as that described aboveexcept that the protective layer is not provided, are shown by thedotted line. As the piezoelectric substrate, 36° LiTaO₃ is used, and asa metal forming the IDTs 5 and 6, aluminum (Al) is used. In addition,when a wavelength of a surface acoustic wave is represented by λ, athickness h of the SiO₂ film is set such that h/λ=0.30 is satisfied.

As shown in FIG. 12B, by providing the protective layer 11, it isunderstood that while the insertion loss characteristics are notsignificantly deteriorated, the spurious outside the pass band can beeffectively decreased. In addition, by providing the protective layerdescribed above, as compared to a surface acoustic wave filter having noprotective layer, temperature coefficient of frequency can also bedecreased.

In FIG. 13, deterioration of the insertion loss characteristics of thesurface acoustic wave filter is shown when the IDT thickness is changed.As shown in FIG. 13, when a wavelength of a surface acoustic wave filterwhich is to be used is represented by λ, and a thickness H of the IDTelectrode is set such that H/λ is preferably about 0.06 or less and ismore preferably about 0.045 or less, the deterioration of the insertionloss characteristics can be prevented.

In the case shown in FIG. 13, Al is preferably used for the IDT;however, when copper (Cu), gold (Au), or other suitable material is usedfor the IDT, the inventors of the present invention confirmed that thesame advantages as those described above could be obtained.

In the end-surface reflection type surface acoustic wave filter inaccordance with the first preferred embodiment of the present invention,although the side surfaces of the first and the second grooves or thefirst and the second recess portions, which are disposed in thepiezoelectric substrate at the IDT side, are used as reflectionend-surfaces, the flexible resin-coating layer is arranged so as tocover at least the region at which the IDTs are located, and at leastone of the first and the second grooves or at least one of the first andthe second recess portions, and the flexible resin-coating layerintrudes into one of the grooves or one of the recess portions.Accordingly, while the insertion loss characteristics are not seriouslydeteriorated, the attenuation amount outside the pass band can beincreased. As a result, a compact surface acoustic wave filter which hassuperior degree of selectivity and which does not require reflectors canbe provided.

In the end-surface reflection type surface acoustic wave filter inaccordance with the second preferred embodiment of the presentinvention, since the flexible resin-coating layer is arranged so as tocover about 20% or greater of an area of the top surface of thepiezoelectric substrate where the plurality of interdigital transducersare located, as in the case of the first preferred embodiment of thepresent invention, while the insertion loss characteristics are not soseriously deteriorated, the attenuation amount outside the pass band canbe increased. Consequently, a compact surface acoustic wave filter whichhas superior degree of selectivity and which does not require reflectorscan be provided.

In the end-surface reflection type surface acoustic wave filter inaccordance with the third preferred embodiment of the present invention,since the layer made of SiO₂ is disposed on the top surface of thepiezoelectric substrate so as to cover the IDTs, as in the cases of thefirst and the second preferred embodiments of the present invention,while the insertion loss characteristics are not so seriouslydeteriorated, the attenuation amount outside the pass band can beincreased. Consequently, a compact surface acoustic wave filter whichhas superior degree of selectivity and which does not require reflectorscan be provided.

When the depth of the first and the second grooves or the first and thesecond recess portions is substantially equivalent to or larger than awavelength of an SH type surface acoustic wave which is to be used, theSH type surface acoustic wave to be used is reliably reflected on theside surfaces of the first and the second grooves or the first and thesecond recess portions at the IDT side, and hence an end-surfacereflection type surface acoustic wave filter having superior propertiescan be obtained.

While preferred embodiments of the invention have been described above,it is to be understood that variations and modifications will beapparent to those skilled in the art without departing the scope andspirit of the invention. The scope of the invention, therefore, is to bedetermined solely by the following claims.

What is claimed is:
 1. An end-surface reflection type surface acousticwave filter comprising: a piezoelectric substrate including first andsecond reflection end-surfaces disposed substantially parallel to eachother and spaced from each other by a predetermined distance, a topsurface, a bottom surface, first and second piezoelectric substrateportions which extend from bottom ends of the first and the secondreflection end-surfaces to outside of the filter, and first and secondgrooves or first and second recess portions open to the outside of thefilter, the first and the second reflection end-surfaces and the firstand the second piezoelectric substrate portions defining the first andthe second grooves or the first and the second recess portions,respectively; a plurality of interdigital transducers disposed on thetop surface of the piezoelectric substrate between the first and thesecond grooves or between the first and the second recess portions; anda resin-coating layer which is provided on the top surface of thepiezoelectric substrate so as to cover at least said plurality ofinterdigital transducers and regions at which the first and the secondgrooves or the first and the second recess portions are located andwhich includes a flexible resin; wherein the resin-coating layerpartially intrudes into at least one of the first and the second groovesor at least one of the first and the second recess portions.
 2. Anend-surface reflection type surface acoustic wave filter according toclaim 1, wherein the flexible resin has a density of about 1.2 g/cm³ orless after being cured.
 3. An end-surface reflection type surfaceacoustic wave filter according to claim 2, wherein the flexible resinhas a density of about 1.0 g/cm³ or less after being cured.
 4. Anend-surface reflection type surface acoustic wave filter according toclaim 1, wherein the flexible resin has a linear expansion coefficientof about 1.9×10⁻⁴ (1/° C.) or greater after being cured.
 5. Anend-surface reflection type surface acoustic wave filter according toclaim 4, wherein the flexible resin has a linear expansion coefficientof about 2.3×10⁻⁴ (1/° C.) or greater after being cured.
 6. Anend-surface reflection type surface acoustic wave filter according toclaim 1, wherein the flexible resin is one of a silicone rubber, anepoxy resin, and a urethane rubber.
 7. An end-surface reflection typesurface acoustic wave filter according to claim 6, wherein the flexibleresin is a silicone rubber.
 8. An end-surface reflection type surfaceacoustic wave filter according to claim 1, wherein the flexible resinhas a Shore hardness of about 30 or less.
 9. An end-surface reflectiontype surface acoustic wave filter according to claim 1, wherein theflexible resin is a gel resin.
 10. An end-surface reflection typesurface acoustic wave filter according to claim 1, wherein the flexibleresin has a Young's modulus of about 1 MPa or less after being cured.11. An end-surface reflection type surface acoustic wave filteraccording to claim 1, wherein the flexible resin is a gel resin, andafter being cured, the flexible resin has a density of about 1.0 g/cm³or less, a Young's modulus of about 1 MPa or less, and a linearexpansion coefficient of about 2.3×10⁻⁴ (1/° C.) or greater.
 12. Anend-surface reflection type surface acoustic wave filter according toclaim 1, wherein each of the grooves or each of the recess portions hasa depth that is substantially equal to or greater than a wavelength ofan SH type surface acoustic wave which is to be used.
 13. An end-surfacereflection type surface acoustic wave filter according to claim 1,wherein the end-surface reflection type surface acoustic wave filter isa ladder type filter which includes an end-surface reflection resonator.14. An end-surface reflection type surface acoustic wave filteraccording to claim 1, wherein the end-surface reflection type surfaceacoustic wave filter is a longitudinally coupled resonator filter. 15.An end-surface reflection type surface acoustic wave filter according toclaim 1, wherein the end-surface reflection type surface acoustic wavefilter is a transversely coupled resonator filter.